Processes for converting lower alcohols such as methanol to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroliferous origin. In particular, they provide a way by which methanol can be converted to gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes.
The conversion of methanol to hydrocarbon products may take place in a fluidized bed process as described, for example, in U.S. Pat. Nos. 4,071,573 and 4,138,440, or in a fixed bed as described in U.S. Pat. Nos. 3,998,899, 3,931,349 and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed over a catalyst such as zeolite ZSM-5 which brings about the conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. The water may be removed from the methanol dehydration products prior to conversion to hydrocarbons as may the methanol which can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of the water vapor at the reaction temperatures employed, but this step is by no means essential.
In the operation of the fixed bed process, a major problem which has to be dealt with is the thermal balance. The conversion of the oxygenated feed stream (methanol, DME) to the hydrocarbons is a strongly exothermic reaction liberating approximately 1480 kJ. (1400 Btu) of heat per kilogram of methanol. In an adiabatic reactor this would result in a temperature rise which would lead to extremely fast catalyst aging rates or even to damage to the catalyst. Furthermore, the high temperatures which might occur could cause undesirable products to be produced or the product distribution could be unfavorably changed. It is therefore necessary that some method should be provided to maintain the catalyst bed within desired temperature limits by dissipating the heat of the reaction.
A degree of control over the temperature of the catalyst bed can be achieved by suitable choice of bed configuration but this expedient is generally insufficient by itself and other methods must be employed. One particularly efficaceous method is to employ a light gas portion of the hydrocarbon product as recycle, as described in U.S. Pat. No. 3,931,349. The cooled light hydrocarbon gas (C.sub.4-) is separated from the products and is compressed and reheated before being mixed with the reactant stream entering the bed of conversion catalyst. Although effective in controlling bed temperature, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion, indicating that a reduction in recycle ratio would be economically desirable. The recycle ratio can indeed be decreased but only at the expense of certain disadvantages. Not only will the temperature rise across the catalyst bed be greater, thereby increasing the aging rate of the catalyst but, in addition, the reactor must be operated at a lower and generally less favorable temperature: the outlet temperature must be lowered in order to protect the catalyst from the increased partial pressure of the water which is consequent upon the lower partial pressure of the recycle gas and the inlet temperature must be lowered even further in order to compensate for the greater temperature rise across the catalyst bed. This is generally undesirable because the octane number of the gasoline product is related to reactor temperature with the higher octane products being produced at the higher temperatures. There is also a minimum reactor inlet temperature that must be maintained for the conversion to proceed and consequently, there is a limit on the extent to which the recycle ratio can be reduced. Lower temperatures also bring about an increase in the production of durene although the product can be treated to reduce the proportion of this undesirable product, albelt at extra expense.
An alternative proposal is set out in U.S. Pat. No. 4,035,430. The process described in this patent employs a number of sequential catalyst beds and recycle gas may be injected between the successive beds to control the exotherm. In addition, an interbed injection of methanol or DME may be used as a quench so as to maintain the temperature rise in each bed to about 28.degree. C. (50.degree. F.) with a total rise over the beds to about 110.degree. C. (200.degree. F.). If this is done, however, the conversion in each reactor must be limited to a relatively low value or, alternatively, an extraordinarily high recycle ratio must be used. The first possibility is obviously unattractive and so is the second, for the reasons set out above. It would therefore be desirable to find a way of carrying out the conversion at desirable temperatures with reduced recycle ratios. Such a method is provided by the present invention.